Heat-resistant structural epoxy resins

ABSTRACT

Epoxy adhesive compositions containing a rubber-modified epoxy resin contain a bisphenol. The bisphenol can be pre-reacted with the rubber-modified epoxy resin to advance the resin. The adhesives are resistant to thermal degradation as can occur in so-called “overbake” conditions, in which the adhesive is heated to high temperatures for prolonged periods of time. In addition, expanded microballoons are included in epoxy structural adhesives to promote a desired fracture mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application no. 12/099,804, filed 9Apr. 2008, now U.S. Pat. No. 8,097,119, and claims priority from UnitedStates Provisional Patent Application No. 60/922,877, filed 11 Apr.2007.

BACKGROUND OF THE INVENTION

This invention relates to an epoxy resin based adhesive.

Epoxy resin based adhesives are used to bond a variety of differentsubstrates together. In the automotive industry, epoxy resin adhesivesare used to bond certain parts together. Structural adhesives areincreasingly being used for metal-metal bonding in frame and otherstructures in automobiles. Adhesive bonding can reduce the number ofwelds that are needed to construct the frame, and for that reason theuse of these adhesives can reduce assembly costs.

Structural adhesives for these applications are subject to verystringent performance requirements. They need to adhere well to metalsthat are commonly used in automotive frame structures, most commonlycold rolled steel (CRS), various types of galvanized metals, and in somecases aluminum. Any of these substrate materials may be coated with oneor more coating materials, including a cationically deposited primermaterial which is commonly known as an “E-coat”. The adhesive should beresistant to impact forces that can be experienced in collisions.Adhesives of this type are sometimes referred to as “crash durableadhesives”, or “CDAs”. In addition, the adhesive must maintain goodbonding to the substrate and good impact resistance over a temperaturerange that can be as low as −40° C. and up to 80° C.

Impact properties are imparted to the adhesive composition through theuse of various rubbers and tougheners. The rubber is typically a liquidpolymer or copolymer of a diene monomer, such as abutadiene-acrylonitrile copolymer of the type described in U.S. Pat. No.6,586,089 and U.S. Published Patent Application No. 2005/0070634. Acommonly used toughener is a capped polyurethane and/or polyurea asdescribed in U.S. Pat. No. 5,278,257.

In the automotive assembly process, the frame is typically bonded andwelded together, and then subjected to one or more coating steps.Automotive coatings are often baked in order to cure them to produce ahard finish. The baking temperatures can range from 140° C. to over 200°C. It has been found that exposure to these temperatures can cause adeterioration of impact properties and adhesion, particularly if theexposure time to these temperatures is long. This tends to be moreapparent at low temperatures. It would be desirable to provide astructural adhesive which bonds well to various metals (including coldrolled steel and various galvanized metal substrates) over a widetemperature range, and which is more resistant to exposure to hightemperatures.

Another desired characteristic of these structural adhesives is the modeof failure. Cohesive failure (i.e., failure within adhesive layer ratherthan at the adhesive-substrate interface or by delamination of thesubstrate) is the preferred failure mode. In many cases, measures thatimpart the desired failure mode tend to reduce impact resistance. Itwould be desirable to provide an adhesive which mainly fails cohesivelywhen bonded to metals such as CRS, galvanized metals and aluminum, andwhich has good impact properties, particularly at low temperature.

SUMMARY OF THE INVENTION

This invention is a one-component structural adhesive, comprising:

-   A) one or more rubber-modified epoxy resins, in which at least a    portion of the rubber has a glass transition temperature of −40° C.    or lower; and-   B) one or more bisphenol compounds,-   wherein component B) may be pre-reacted with all or a portion of    component A) to form an advanced rubber-modified epoxy resin, the    structural adhesive further comprising:-   C) a reactive toughener; and-   D) one or more epoxy curing agents;-   wherein the structural adhesive is curable at a temperature of    80° C. or higher.

The structural adhesive of the invention, when cured, is surprisinglyresistant to high temperatures. The bisphenol compound can be dissolvedinto the structural adhesive composition, or dispersed therein in theform of fine particles. Alternatively, the bisphenol compound can bepre-reacted with all or a portion of the rubber-modified epoxy resins toform an advanced material, which can be used as a component in thestructural adhesive composition. In either case, the cured structuraladhesive is more resistant to degradation of its bonding properties uponexposure to elevated temperatures. These benefits are seen especially inthe low temperature properties of the adhesive, which are well retainedafter the structural adhesive has been exposed to elevated temperatureconditions.

This invention is also a one-component structural adhesive, comprising:

-   A) one or more rubber-modified epoxy resins, in which at least a    portion of the rubber has a glass transition temperature of −40° C.    or lower;-   B) one or more reactive tougheners;-   C) from 1 to 5% by weight of the structural adhesive of expanded    microballoons having an average particle size of up to 200 microns    and density of up to 0.2 g/cc; and-   D) one or more epoxy curing agents;-   wherein the structural adhesive is curable at a temperature of    80° C. or higher.

The presence of the expanded microballoons helps the cured adhesive toachieve cohesive failure when bonded to various substrates, includingCRS and galvanized metals. The expanded microballoons tend to have lessadverse affect on adhesion and impact properties than do otherparticulate fillers.

DETAILED DESCRIPTION OF THE INVENTION

The structural adhesive of the invention includes at least onerubber-modified epoxy resin. Preferably, the rubber-modified epoxy resinis an epoxy-terminated adduct of an epoxy resin and at least one liquidrubber that has epoxide-reactive groups, such as amino or preferablycarboxyl groups. At least a portion of the rubber has a glass transitiontemperature (T_(g)) of −40° C. or lower, preferably −50° C. or lower.Preferably, each of the rubbers (when more than one is used) has a glasstransition temperature of −25° C. or lower. The rubber T_(g) may be aslow as −100° C. or even lower.

The rubber is preferably a homopolymer of a conjugated diene orcopolymer of a conjugated diene, especially a diene/nitrile copolymer.The conjugated diene rubber is preferably butadiene or isoprene, withbutadiene being especially preferred. The preferred nitrile monomer isacrylonitrile. Preferred copolymers are butadiene-acrylonitrilecopolymers. The rubbers preferably contain, in the aggregate, no morethan 30 weight percent polymerized unsaturated nitrile monomer, andpreferably no more than about 26 weight percent polymerized nitrilemonomer.

The rubber preferably contains from about 1.5, more preferably fromabout 1.8, to about 2.5, more preferably to about 2.2, epoxide-reactiveterminal groups per molecule, on average. Carboxyl-terminated rubbersare preferred. The molecular weight (Me) of the rubber is suitably fromabout 2000 to about 6000, more preferably from about 3000 to about 5000.

Suitable carboxyl-functional butadiene and butadiene/acrylonitrilerubbers are commercially available from Noveon under the tradenamesHycar® 2000×162 carboxyl-terminated butadiene homopolymer and Hycar®1300×31 Hycar® 1300×8, Hycar® 1300×13, Hycar® 1300×9 and Hycar® 1300×18carboxyl-terminated butadiene/acrylonitrile copolymers. A suitableamine-terminated butadiene/acrylonitrile copolymer is sold under thetradename Hycar® 1300×21.

The rubber is formed into an epoxy-terminated adduct by reaction with anexcess of an epoxy resin. Enough of the epoxy resin is provided to reactwith all of the epoxide-reactive groups on the rubber and to providefree epoxide groups on the resulting adduct without significantlyadvancing the adduct to form high molecular weight species. A ratio ofat least two equivalents of epoxy resin per equivalent of epoxy-reactivegroups on the rubber is preferred. More preferably, enough of the epoxyresin compound is used that the resulting product is a mixture of theadduct and some free epoxy resin compound. Typically, the rubber and anexcess of the epoxy resin are mixed together with a polymerizationcatalyst and heated to a temperature of about 100 to about 250° C. inorder to form the adduct. Useful catalysts for conducting the reactionbetween the rubber and the epoxy resin include those described below.Preferred catalysts for forming the rubber-modified epoxy resin includephenyl dimethyl urea and triphenyl phosphine.

A wide variety of epoxy resins can be used to make the rubber-modifiedepoxy resin, including any of those described below. Preferred epoxyresins are liquid or solid glycidyl ethers of a bisphenol such asbisphenol A or bisphenol F. Halogenated, particularly brominated, resinscan be used to impart flame retardant properties if desired. Liquidepoxy resins (such as DER 330 and DER 331 resins, which are diglycidylethers of bisphenol A available from The Dow Chemical Company) areespecially preferred for ease of handling.

The rubber-modified epoxy resin(s) preferably constitute at least about4 weight percent of the structural adhesive, more preferably at leastabout 5 weight percent. The rubber-modified epoxy resin may constituteup to about 30 weight percent of the structural adhesive, morepreferably up to about 20 weight percent, and even more preferably up toabout 15 weight percent.

The bisphenol component is any compound having two or more, preferablytwo, phenolic hydroxyl groups per molecule. The bisphenol componentpreferably has a molecular weight of 500 or less, preferably 300 orless. Examples of suitable bisphenol compounds include, for example,resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP(1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenol F, bisphenol K,tetramethylbiphenol and the like.

The bisphenol component can be dissolved into the structural adhesivecomposition or present in the form of finely divided particles.Preferably, the bisphenol component is pre-reacted with one or more ofthe rubber-modified epoxy resins to advance the resin somewhat. In someembodiments of the invention, a mixture of rubber-modified epoxy resinsis used. In such cases, it is normally necessary to advance only one ofthe rubber-modified epoxy resins, although any number of them (or all ofthem) can be advanced in this manner. In some embodiments, a mixture ofrubber-modified epoxy resins will include at least one component whichis prepared from a rubber having a T_(g) of greater than −40° C., andanother rubber having a T_(g) of −40° C. or below. In such cases, eitheror both of the rubber-modified epoxy resins may be advanced.

The bisphenol component is preferably used in an amount from about 3 toabout 35 parts by weight per 100 parts by weight of the rubber componentin the rubber-modified epoxy resin. A preferred amount is from about 5to about 25 parts by weight per 100 parts by weight of the rubbercomponent of the rubber-modified epoxy resin. When the bisphenolcomponent is added directly into the structural adhesive, it usuallyconstitutes from 0.25 to 2 weight percent, especially 0.4 to 1.5 weightpercent, of the adhesive.

The reactive toughener is a liquid or low-melting elastomeric materialhaving capped or blocked isocyanate, amino, hydroxyl or thiol groups. Itshould be soluble or dispersible in the remainder of the reactivecomponents of the structural adhesive. Isocyanate groups can be cappedor blocked with, for example, a phenolic compound, an aminophenoliccompound, a primary or secondary aliphatic or cycloaliphatic amine, abenzyl alcohol, an aromatic amine, a benzyl amine or a thiol compound.Hydroxyl and thiol groups can be capped or blocked with, for example, anisocyanate compound, a cyclic carbonate or urethane, an aromatichydroxycarboxylic acid or an aromatic aminocarboxylic acid. Amino groupscan be capped, for example, with an isocyanate compound or an anhydridesuch as isatoic anhydride. In all cases, the capping or blocking groupmay contain functional groups such as phenol, aromatic amino, —OCN,epoxide, or like groups, but the capping or blocking group may insteadbe devoid of such groups. Tougheners of these types and methods forpreparing them are described, for example, in U.S. Pat. No. 5,202,390,U.S. Pat. No. 5,278,257, WO 2005/118734, U.S. Published PatentApplication No. 2005/0070634, U.S. Published Patent Application No.2005/0209401 and U.S. Published Patent Application 2006/0276601. Theelastomeric portion of the reactive toughener advantageously includes apolyether, polyamine or polyester segment. The polyether, polyamine orpolyester segment may form part of a polyurethane and/or polyureabackbone.

The reactive toughener preferably has a viscosity at 45° C. which is notgreater than 1000 Pa·s and more preferably no more than about 800 Pa·s.Preferably, the weight average molecular weight of the toughener isabout 8,000 or greater, and more preferably about 10,000 or greater.Preferably, the weight average molecular weight of the toughener isabout 70,000 or less, and more preferably about 40,000 or less.Molecular weights as used herein are determined according to GPCanalysis.

The reactive toughener preferably contains an average no more than 6blocked or capped terminal groups per molecule. Preferably the averagenumber of such groups is at least 1, more preferably at least 2, up toabout 4 per molecule.

The toughener is preferably non-crosslinked or lightly crosslinked,preferably having a crosslink density of about 2 or less and preferablyabout 1 or less. Crosslink density is the number of attachments betweenchains of polymers.

A preferred class of tougheners include those corresponding to FormulaI:

wherein m is 1 or 2, n is 2 to 6, R¹ is an n-valent radical of anelastomeric prepolymer after the removal of the terminal isocyanate,amino or hydroxyl group(s), the elastomeric prepolymer being soluble ordispersible in epoxy resin, W and X are independently —O— or —NR³—, atleast one of W and X being —NR³—, R² is an m+1-valent radical of apolyphenol or aminophenol after the removal of the phenolic hydroxylgroup when X is —O— and of the amino group when X is —NR³—, and R³ ishydrogen, a C₁ to C₆ alkyl or phenyl. Such tougheners are described inmore detail in EP-A-0 308 664 (page 5, line 14, to page 13, line 24),and in U.S. Pat. No. 5,278,257 (at column 2, lines 14 to 33 and column4, line 19 and column 16, line 18), the disclosures of which areincorporated herein by reference.

Another suitable toughener corresponds to Formula II:

wherein p is 1 or 2, n is 2 to 6, Y′ is —O—, —S— or —NR⁶—, Z is aradical selected from the group consisting of —OH, —NHR⁶, —OCN,

R⁴ is a residue of a hydroxyl-, mercapto- or amino-terminated polyetherprepolymer or of a hydroxyl-, mercapto- or amino-terminatedprepolymeric, segmented polyester, polythioester or polyamide, R⁵ is acarbocyclic aromatic or araliphatic p+1-valent radical with groups Zbonded directly to the aromatic ring, R⁶ is hydrogen, C₁ to C₆ alkyl orphenyl, and R⁷ is methyl or hydrogen. Tougheners corresponding toFormula II are described in more detail in EP-A-0 353 190 (page 3, line51, to page 6, line 62) and U.S. Pat. No. 5,202,390 (at column 1, line59 to column 2, line 16 and column 3, line 60 to column 9, line 21), thedisclosures of which are incorporated herein by reference.

Still other suitable reactive tougheners correspond to Formula IIIand/or Formula IV

wherein R⁸ is independently in each occurrence a C₂₋₂₀ m-valent alkylmoiety; R⁹ is independently in each occurrence a polyether chain; R¹⁰ isindependently in each occurrence an alkylene, cycloalkylene or mixedalkylene and cycloalkylene moiety, optionally containing one or moreoxygen or sulfur atoms; R¹¹ is a direct bond or an alkylene, carbonyl,oxygen, carboxyloxy, or amido moiety; R¹² is independently in eachoccurrence an alkyl, alkenyl, alkoxy, aryloxy or aryloxy moiety with theproviso that if s=1, then q=0; X′ is O or —NR¹³ with the proviso that X′is O where s is 1; and that where s is 0, X′ is O in at least oneoccurrence; R¹³ is independently in each occurrence hydrogen or alkyl; tis independently in each occurrence a number of about 1 to about 6; u isindependently in each occurrence a number of 1 or greater; o isindependently in each occurrence 0 or 1 if s is 0 and 0 if s is 1; s isindependently in each occurrence 0, or 1; and q is independently in eachoccurrence a number of from 0 to 1.

Still another useful class of reactive tougheners corresponds to formulaV:

wherein R¹⁴ is the elastomeric prepolymer residue, said residue having avalence of t+v=2 to 6 with t=1 to 6 and v=0 to 5, X″ is the residue ofthe primary aliphatic, cycloaliphatic, heteroaromatic and/or araliphaticamine and/or the secondary aliphatic, cycloaliphatic, aromatic,heteroaromatic and/or araliphatic amine, the thiol and/or the alkylamide and Y″ is the residue of the phenol and/or the polyphenol.

Preferred reactive tougheners are isocyanate-terminated prepolymersformed from a polyether polyol and an aliphatic polyisocyanate, in whichthe terminal isocyanate groups are blocked with a phenol, aminophenol,polyphenol or an allylphenol, such as o,o-diallyl bisphenol A.

The reactive toughener is present in sufficient amount to improve theperformance of adhesive compositions containing it under dynamic load.Preferably, the reactive toughener constitutes at least about 5 weightpercent of the structural adhesive, preferably at least about 8 weightpercent and most preferably at least about 12 weight percent.Preferably, the reactive toughener constitutes up to about 40 weightpercent of the structural adhesive, preferably up to about 30 weightpercent and more preferably up to about 25 weight percent.

The structural adhesive further contains a curing agent. The curingagent is selected together with any catalysts such that the adhesivecures when heated to a temperature of 80° C., preferably at least 100°C. or greater, but cures very slowly if at all at room temperature (˜22°C.) and at temperatures up to at least 50° C. Suitable such curingagents include boron trichloride/amine and boron trifluoride/aminecomplexes, dicyandiamide, melamine, diallylmelamine, guanamines such asacetoguanamine and benzoguanamine, aminotriazoles such as3-amino-1,2,4-triazole, hydrazides such as adipic dihydrazide, stearicdihydrazide, isophthalic dihydrazide, semicarbazide, cyanoacetamide, andaromatic polyamines such as diaminodiphenylsulphones. The use ofdicyandiamide, isophthalic acid dihydrazide, adipic acid dihydrazide and4,4′-diaminodiphenylsulphone is particularly preferred.

The curing agent is used in sufficient amount to cure the composition.The curing agent suitably constitutes at least about 1.5 weight percentof the structural adhesive, and more preferably at least about 2.5weight percent. The curing agent preferably constitutes up to about 15weight percent of the adhesive composition, more preferably up to about10 weight percent, and most preferably about 6 weight percent.

The structural adhesive will in most cases contain a catalyst for thecure of the adhesive. Among preferred epoxy catalysts are ureas such asp-chlorophenyl-N,N-dimethylurea (Monuron), 3-phenyl-1,1-dimethylurea(Phenuron), 3,4-dichlorophenyl-N,N-dimethylurea (Diuron),N-(3-chloro-4-methylphenyl)-N′,N′-dimethylurea (Chlortoluron),tert-acryl- or alkylene amines like benzyldimethylamine,2,4,6-tris(dimethylaminomethyl)phenol, piperidine or derivates thereof,imidazole derivates, in general C₁-C₁₂ alkylene imidazole orN-arylimidazols, such as 2-ethyl-2-methylimidazol, or N-butylimidazol,6-caprolactam, a preferred catalyst is2,4,6-tris(dimethylaminomethyl)phenol integrated into apoly(p-vinylphenol) matrix (as described in European patent EP 0 197892). The catalyst may be encapsulated or otherwise be a latent typewhich becomes active only upon exposure to elevated temperatures.Preferably, the catalyst is present in the adhesive composition in theamount of at least about 0.1 weight percent of the structural adhesive,and most preferably about 0.2 weight percent. Preferably, the epoxycuring catalyst is present in an amount of up to about 2 weight percentof the structural adhesive, more preferably up to about 1.0 weightpercent, and most preferably about 0.7 weight percent.

The structural adhesive of the invention may contain various optionalcomponents. Among these, fillers, rheology modifiers or pigments, one ormore additional epoxy resins and a core-shell rubber are particularlypreferred.

A filler, rheology modifier and/or pigment is preferably present in thestructural adhesive. These can perform several functions, such as (1)modifying the rheology of the adhesive in a desirable way, (2) reducingoverall cost, (3) absorbing moisture or oils from the adhesive or from asubstrate to which it is applied, and/or (4) promoting cohesive, ratherthan adhesive, failure. Examples of these materials include calciumcarbonate, calcium oxide, talc, coal tar, carbon black, bitumen, textilefibers, glass particles or fibers, asbestos fibers, boron fibers, carbonfibers, mineral silicates, mica, powdered quartz, hydrated aluminumoxide, bentonite, wollastonite, kaolin, fumed silica, silica aerogel ormetal powders such as aluminum powder or iron powder. Among these,calcium carbonate, talc, calcium oxide, fumed silica and wollastoniteare preferred, either singly or in some combination, as these oftenpromote the desired cohesive failure mode.

A filler of particular interest is a microballoon having an averageparticle size of up to 200 microns and density of up to 0.2 g/cc. Theparticle size is preferably about 25 to 150 microns and the density ispreferably from about 0.05 to about 0.15 g/cc. Expanded microballoonswhich are suitable include those commercially available from DualiteCorporation under the trade designation Dualite™, and also from Lehmann& Voss, Hamburg, Germany. Specific examples of suitable polymericmicroballoons include Dualite™ E065-135 and Dualite E130-40Dmicroballoons. Microballoons of the type have been found to stronglypromote the desired cohesive failure when used at a level of from about1 to about 5 weight percent, preferably 1.5 to 3 weight percent, of thestructural adhesive. Microballoons are preferably used in conjunctionwith one or more additional fillers, such as talc, calcium oxide,wollastonite, calcium carbonate, fumed silica or mixtures thereof.

Fillers, rheology modifiers and pigments preferably are used in anaggregate amount of about 5 parts per hundred parts of adhesivecomposition or greater, more preferably about 10 parts per hundred partsof adhesive composition or greater. They preferably are present in anamount of up to about 25 weight percent of the structural adhesive, morepreferably up to about 20 weight percent, and most preferably up toabout 15 weight percent.

The structural adhesive preferably contains at least one epoxy resinwhich is not rubber-modified, in addition to the rubber-modified epoxyresin described before. As noted, in some instances the rubber-modifiedepoxy resin may contain some quantity of epoxy resin which is notreacted with the rubber, and some amount of additional epoxy resin maybe brought into the structural adhesive in that manner. Alternatively orin addition, the additional epoxy resin may be separately added to thestructural adhesive.

The additional epoxy resin may be the same or different from that usedto prepare the rubber-modified epoxy resin. A wide range of epoxy resinscan be used as the additional epoxy resin including those described atcolumn 2 line 66 to column 4 line 24 of U.S. Pat. No. 4,734,332,incorporated herein by reference.

Suitable epoxy resins include the diglycidyl ethers of polyhydric phenolcompounds such as resorcinol, catechol, hydroquinone, bisphenol,bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane),bisphenol F, bisphenol K, bisphenol M, tetramethylbiphenol, diglycidylethers of aliphatic glycols and polyether glycols such as the diglycidylethers of C₂₋₂₄ alkylene glycols and poly(ethylene oxide) orpoly(propylene oxide) glycols; polyglycidyl ethers ofphenol-formaldehyde novolac resins, alkyl substitutedphenol-formaldehyde resins (epoxy novalac resins),phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins,dicyclopentadiene-phenol resins and dicyclopentadiene-substituted phenolresins, and any combination thereof.

Suitable diglycidyl ethers include diglycidyl ethers of bisphenol Aresins such as are sold by Dow Chemical under the designations D.E.R.®330, D.E.R.® 331, D.E.R.® 332, D.E.R.® 383, D.E.R. 661 and D.E.R.® 662resins.

Commercially available diglycidyl ethers of polyglycols include thosesold as D.E.R.® 732 and D.E.R.® 736 by Dow Chemical.

Epoxy novolac resins can be used. Such resins are available commerciallyas D.E.N.® 354, D.E.N.® 431, D.E.N.® 438 and D.E.N.® 439 from DowChemical.

Other suitable additional epoxy resins are cycloaliphatic epoxides. Acycloaliphatic epoxide includes a saturated carbon ring having an epoxyoxygen bonded to two vicinal atoms in the carbon ring, as illustrated bythe following structure III:

wherein R is an aliphatic, cycloaliphatic and/or aromatic group and n isa number from 1 to 10, preferably from 2 to 4. When n is 1, thecycloaliphatic epoxide is a monoepoxide. Di- or epoxy resins are formedwhen n is 2 or more. Mixtures of mono-, di- and/or epoxy resins can beused. Cycloaliphatic epoxy resins as described in U.S. Pat. No.3,686,359, incorporated herein by reference, may be used in the presentinvention. Cycloaliphatic epoxy resins of particular interest are(3,4-epoxycyclohexyl-methyl)-3,4-epoxy-cyclohexane carboxylate,bis-(3,4-epoxycyclohexyl)adipate, vinylcyclohexene monoxide and mixturesthereof.

Other suitable epoxy resins include oxazolidone-containing compounds asdescribed in U.S. Pat. No. 5,112,932. In addition, an advancedepoxy-isocyanate copolymer such as those sold commercially as D.E.R. 592and D.E.R. 6508 (Dow Chemical) can be used.

The additional epoxy resin preferably is a bisphenol-type epoxy resin ormixture thereof with up to 10 percent by weight of another type of epoxyresin. Preferably the bisphenol type epoxy resin is a liquid epoxy resinor a mixture of a solid epoxy resin dispersed in a liquid epoxy resin.The most preferred epoxy resins are bisphenol-A based epoxy resins andbisphenol-F based epoxy resins.

The additional epoxy resin or epoxide resin is used in sufficient amountto impart desirable adhesive and strength properties. Preferably, theadditional epoxy resin constitutes at least about 10 weight percent ofthe structural adhesive, more preferably at least about 15 weightpercent, and most preferably at least about 20 weight percent. Theadditional epoxy resin preferably constitutes up to about 60 weightpercent of the structural adhesive, more preferably up to about 50weight percent, and most preferably up to about 40 weight percent.

Another highly preferred but optional component is a core-shell rubber.

Core-shell rubbers are well known. They contain at least one coreportion which is a rubbery material preferably having a T_(g) of lessthan −10° C., more preferably less than −40° C., even more preferablyless than −50° C. and still more preferably less than −70° C., and atleast one shell portion that preferably has a T_(g) of at least 50° C.The T_(g) of the rubbery core may be well below −100° C. By “core”, itis meant an internal portion of the core-shell rubber. The core may formthe center of the core-shell particle, or an internal shell or domain ofthe core-shell rubber. A shell is a portion of the core-shell rubberthat is exterior to the rubbery core. The shell portion (or portions)typically forms the outermost portion of the core-shell rubber particle.The shell material is preferably grafted onto the core or iscrosslinked. The rubbery core may constitute from 50 to 95%, especiallyfrom 60 to 90% of the weight of the core-shell rubber particle.

The core of the core-shell rubber may be a polymer or copolymer of aconjugated diene such as butadiene, or a lower alkyl acrylate such asn-butyl-, ethyl-, isobutyl- or 2-ethylhexylacrylate. The core polymermay in addition contain up to 20% by weight of other copolymerizedmonounsaturated monomers such as styrene, vinyl acetate, vinyl chloride,methyl methacrylate, and the like. The core polymer is optionallycrosslinked. The core polymer also optionally contains up to 5% of acopolymerized graft-linking monomer having two or more sites ofunsaturation of unequal reactivity, such as diallyl maleate, monoallylfumarate, allyl methacrylate, and the like, at least one of the reactivesites being non-conjugated.

The core polymer may also be a silicone rubber. These materials oftenhave glass transition temperatures below −100° C. Core-shell rubbershaving a silicone rubber core include those commercially available fromWacker Chemie, Munich, Germany, under the trade name Genioperl™.

The shell polymer, which is optionally chemically grafted or crosslinkedto the rubbery core, is preferably polymerized from at least one loweralkyl methacrylate such as methyl-, ethyl- or t-butyl methacrylate.Homopolymers of such methacrylate monomers can be used. Further, up to40% by weight of the shell polymer can be formed from othermonovinylidene monomers such as styrene, vinyl acetate, and vinylchloride, methyl acrylate, ethyl acrylate, butyl acrylate, and the like.The molecular weight of the grafted shell polymer is generally between20,000 and 500,000.

A preferred type of core-shell rubber has reactive groups in the shellpolymer which can react with an epoxy resin or an epoxy resin hardener.Glycidyl groups such as are provided by monomers such as glycidylmethacrylate are suitable.

A particularly preferred type of core-shell rubber is of the typedescribed in EP 1 632 533 A1. Core-shell rubber particles as describedin EP 1 632 533 A1 include a crosslinked rubber core, in most casesbeing a crosslinked copolymer of butadiene, and a shell which ispreferably a copolymer of styrene, methyl methacrylate, glycidylmethacrylate and optionally acrylonitrile. The core-shell rubber ispreferably dispersed in a polymer or an epoxy resin, also as describedin EP 1 632 533 A1.

Preferred core-shell rubbers include those sold by Kaneka Corporationunder the designation Kaneka Kane Ace, including Kaneka Kane Ace MX 156and Kaneka Kane Ace MX 120. These materials are pre-dispersed in anepoxy resin, which becomes an additional epoxy resin as describedbefore.

The core-shell rubber particles can constitute from 0 to 30 weightpercent of the structural adhesive. The particles preferably constituteat least 1 weight percent and more preferably at least 1.25 weightpercent of the structural adhesive. The core-shell rubber particlespreferably constitute no more than 10 weight percent, more preferably nomore than 5 weight percent of the structural adhesive.

The amount of core-shell rubber particles(if any) and the amount ofrubber-modified epoxy resin are selected together such that the totalrubber content of the structural adhesive is at least 4 weight percent.The total rubber content may be as much as 30 weight percent, preferablyfrom 6 to 20 weight percent and more preferably from 8 to 15 weightpercent of the structural adhesive.

Total rubber content is calculated for purposes of this invention bydetermining the weight of core-shell rubber particles (if any), plus theweight contributed by the liquid rubber portion of the rubber-modifiedepoxy resin(s). In each case, the weight of unreacted(non-rubber-modified) epoxy resins and/or other carriers, diluents,dispersants or other ingredients that may be contained in the core-shellrubber product or rubber-modified epoxy resin is not included. Theweight of the shell portion of the core-shell rubber is counted as partof the total rubber content for purposes of this invention.

Particularly preferred structural adhesives of the invention have atotal rubber content of at least 6 weight percent, especially from 8 to20 weight percent, and also contain 10% or more, especially 12% or more,of the reactive toughener.

The structural adhesive can further contain other additives such asdiluents, plasticizers, extenders, pigments and dyes, fire-retardingagents, thixotropic agents, flow control agents, such as silicones,waxes and stearates, which can, in part, also be used as mold releaseagents, adhesion promoters, antioxidants and light stabilizers.

Especially preferred structural adhesives have the followingcompositions:

Component Weight Percent Core-shell rubber particles, T_(g) of rubber0-10, preferably 1.25-5 portion <−70° C. Rubber-modified epoxy resin(s)4-25, preferably 5-20 Bisphenol component 0.25-2.0, preferably 0.4-1.5Reactive Toughener 5-25, preferably 10-20 Reactive Diluent 0-8 Curingagent 2.5-8, preferably 3-6 Catalyst 0.2-2, preferably 0.4-0.8 Talc 0-5,preferably 0.4-2 Calcium oxide 0-10, preferably 1-8 ExpandedMicroballoons 0-4, preferably 1-2 Wollastonite 0-15, preferably 2-15Calcium Carbonate 2-20, preferably 8-15 Fumed Silica 1-12, preferably2-8 Additional Epoxy Resin 0-60, preferably 15-40 Total Fillers 5-25 %Total Rubber 8-15

The adhesive composition can be applied by any convenient technique. Itcan be applied cold or be applied warm if desired. It can be applied byextruding it from a robot into bead form on the substrate, it can beapplied using mechanical application methods such as a caulking gun, orany other manual application means, it can also be applied using a swirltechnique. The swirl technique is applied using an apparatus well knownto one skilled in the art such as pumps, control systems, dosing gunassemblies, remote dosing devices and application guns. The adhesive maybe applied to the substrate using a streaming process. Generally, theadhesive is applied to one or both substrates. The substrates arecontacted such that the adhesive is located between the substrates to bebonded together.

After application, the structural adhesive is cured by heating to atemperature at which the curing agent initiates cure of the epoxy resincomposition. Generally, this temperature is about 80° C. or above,preferably 100° C. or above. Preferably, the temperature is about 220°C. or less, and more preferably about 180° C. or less.

The adhesive of the invention can be used to bond a variety ofsubstrates together including wood, metal, coated metal, aluminum, avariety of plastic and filled plastic substrates, fiberglass and thelike. In one preferred embodiment, the adhesive is used to bond parts ofautomobiles together or parts to automobiles. Such parts can be steel,coated steel, galvanized steel, aluminum, coated aluminum, plastic andfilled plastic substrates.

An application of particular interest is bonding of automotive framecomponents to each other or to other components. The frame componentsare often metals such as cold rolled steel, galvanized metals, oraluminum. The components that are to be bonded to the frame componentscan also be metals as just described, or can be other metals, plastics,composite materials, and the like.

Adhesion to brittle metals such as galvaneal is of particular interestin the automotive industry. Galvaneal tends to have a zinc-iron surfacethat is somewhat rich in iron content and is brittle for that reason. Aparticular advantage of this invention is that the cured adhesive bondswell to brittle metals such as galvaneal. This is particularly the casewhen the core-shell rubber particles constitute at least 2.5 weightpercent of the structural adhesive, the total rubber content of thestructural adhesive is at least 10 weight % and the structural adhesivecontains at least 10 weight % of the reactive toughener.

Assembled automotive frame members are usually coated with a coatingmaterial that requires a bake cure. The coating is typically baked attemperatures that may range from 140° C. to over 200° C. In such cases,it is often convenient to apply the structural adhesive to the framecomponents, then apply the coating, and cure the adhesive at the sametime the coating is baked and cured.

In automotive applications, thermal degradation of a structural adhesivetends to come from two main causes. The first cause is excessive timeand/or temperature conditions during the cure of the structural adhesive(which may be done at the same time a coating is baked and cured.)Further exposures to elevated temperatures may occur as the bakingprocess may be repeated one or more additional times during the vehiclemanufacturing process, as different coatings are applied and curedduring different stages of manufacture. A good structural adhesive forthese applications therefore should be able to withstand thesetemperatures and still retain good adhesive properties. Adhesiveproperties at cold temperatures (such as −30° C. or −40° C.) are usuallymost affected by heat degradation that occurs during these baking steps.

It has been found that the inclusion of the bisphenol compound, whenused in conjunction with at least one rubber-modified epoxy resin inwhich the resin has a T_(g) of −40° C. or less, tends to make the curedstructural adhesive more resistant to high temperatures, so that thecured adhesive, after exposure to high temperatures, retains more of itsadhesive strength, especially at low temperatures. A useful method forevaluating this is the ISO 11343 Impact Peel Testing method which isdescribed more fully in the examples. A preferred structural adhesive ofthe invention exhibits an impact peel strength of at least 25 N/mm,preferably at least 30 N/mm, when measured at room temperature afterbaking at 190° C. for 60 minutes or more. A preferred structuraladhesive of the invention also exhibits an impact peel strength of atleast 15 N/mm, preferably at least 18 N/mm, when measured at −40° C.after baking at 190° C. for 60 minutes or more.

An especially preferred structural adhesive of the invention alsoexhibits (when cured) mainly cohesive failure when evaluated in a T-Peeltesting method as described below in the Examples. Balancing cohesivefailure with high impact peel strength is usually difficult, asformulation changes that favor cohesive failure, such as the inclusionof higher levels of fillers, often tend to reduce impact peel.

The adhesive composition once cured preferably has a Young's modulus ofabout 1200 MPa as measured according to DIN EN ISO 527-1. Morepreferably, the Young's modulus is about 1400 MPa or greater.Preferably, the cured adhesive demonstrates a tensile strength of about30 MPa or greater, more preferably about 35 MPa or greater, and mostpreferably about 40 MPa or greater. Preferably, the adhesivedemonstrates an elongation of about 3 percent or greater, morepreferably about 5 percent or greater, and most preferably about 9percent or greater as measured according to DIN EN ISO 527-1.Preferably, the lap shear strength of a 1.5 mm thick cured adhesivelayer is about 15 MPa or greater, more preferably about 20 MPa orgreater, and most preferably about 25 MPa or greater measured accordingto DIN EN 1465.

The following examples are provided to illustrate the invention but arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

Products used in the following examples are identified as follows:

DER™ 330 is a liquid diglycidyl ether of bisphenol A, available from TheDow Chemical Company. It has an epoxy equivalent weight of approximately180.

DER™ 331 is a liquid diglycidyl ether of bisphenol A, available from TheDow Chemical Company. It has an epoxy equivalent weight of approximately187.

Struktol 3611 is a reaction product of approximately 50% of a liquiddiglycidyl ether of bisphenol F, 40% of Hycar 1300×13 rubber (acarboxy-terminated butadiene-acrylonitrile copolymer having a T_(g)greater than −40° C., available from Noveon) and from 2 to 5% ofbisphenol A. It is commercially available from Schill & Seilacher.

Struktol™ 3604 is a reaction product of approximately 60% of a liquiddiglycidyl ether of bisphenol A and 40% of Hycar 1300×8 rubber (acarboxy-terminated butadiene-acrylonitrile copolymer having a T_(g) ofabout −52° C., available from Noveon). It is commercially available fromSchill & Seilacher.

Advanced Struktol™ 3604 is the reaction product of Struktol™ 3604 withbisphenol A, at a weight ratio of about 10:0.6.

RAM 965 reactive toughener is an isocyanate-terminated polyurethaneprepolymer prepared from a polyether polyol and an aliphaticdiisocyanate, in which the isocyanate groups are capped with o,o-diallylbisphenol A, and is made as described in Example 13 of EP 308 664.

RAM B reactive toughener is an isocyanate-terminated polyurethaneprepolymer prepared from a polyether polyol and an aliphaticdiisocyanate, in which the isocyanate groups are capped with bisphenolA. It is described as Toughener B in U.S. Published Patent ApplicationNo. 2005/0070634.

Cardura™ E10 is versatic acid monoepoxy ester, available from ShellChemicals.

Synquest Silane A187 is an epoxy silane available from GE Silicones.

Amicure™ CG-1200 is a cyanoguanidine epoxy hardener available from AirProducts and Chemicals.

EP796 is tris (2,4,6-dimethylaminomethyl)phenol in a polymer matrix,available from Huntsman Chemicals.

Dualite™ E065-135 is an expanded microballoon having a bulk density of0.065 g/cc and an average particle size of 135 microns.

Dualite E136-040D is an expanded microballoon having a bulk density of0.13 g/cc and an average particle size of 40 microns.

Nyad™ M400 is a particulate wollastonite from Nyco.

Microthene™ FE53200 is a 20 micron, 8 melt index, 0.926 density poweredethylene vinylacetate copolymer from Equistar Chemicals.

Aerosil™ R202 and Aerosil R805 are particulate fumed silica productsfrom Degussa.

Kaneka Kane Ace MX 156 is a dispersion of about 25% of a core-shellrubber in an epoxy resin. The core-shell rubber has a crosslinkedpolybutadiene core. It is available commercially from KanekaCorporation.

EXAMPLES 1 and 2

Structural adhesive Examples 1 and 2 are prepared from the componentsset forth in Table 1.

TABLE 1 Parts By Weight Example No. Components 1 2 DER 330 Epoxy Resin34.2 34.2 Strutkol 3611 17.1 17.1 Struktol 3604 11.5 11.5 RAM 965 18.918.9 Cardura E10 1.2 1.2 Colorants 0.4 0.4 Synquest Silane A187 0.7 0.7Amicure CG-1200 3.8 3.8 EP796 0.9 0.9 Dualite E065-135 1.0 1.0 Nyad M4000 15.0 Calcium Carbonate 15.0 0 Microthene FE53200 2.0 2.0 Aerosil R2024.0 4.0

The epoxy resin, rubber-modified epoxy resins, toughener and colorants,RAM 1086, Amicure CG-1200 and EP 796 are mixed on a planetary mixturefor about 5 minutes. Waste heat raises the temperature of the mixture toabout 50° C. One-half of each of the remaining ingredients is then addedand the mixture stirred at about 50° C. for about 10 minutes. Theremainder of the components is then added and the resulting mixture isstirred for another 10 minutes at about 50° C. A scrape-down isperformed, and the components are mixed further for 30 minutes undervacuum.

Impact peel testing is performed with each of Examples 1 and 2, inaccordance with ISO 11343 wedge impact method. Testing is performedusing an Instron Dynatup 8250 device operated at 2 mm/sec. The substrateis hot dipped galvanized steel, Advanced Coatings Technology ACT HDG G7070U Cut Only Unpolished grade.

Multiple test samples are prepared using each of Example 1 and Example2.

Test coupons are 100 mm×20 mm with a bonded area of 20×30 mm. Thesamples are prepared by wiping them with acetone. A 0.15 mm×10 mm wideTeflon tape is applied to the coupons to define a 20×30 bond area. Anoil (Quaker Ferrocoate 61 AUS) is applied to one coupon. A lighter levelof oil is applied to the other coupon. The structural adhesive is thenapplied to the bond area of latter coupon and squeezed onto the firstcoupon to prepare each test specimen.

Duplicate test specimens for each of Examples 1 and 2 are baked at 170°C. for 30 minutes. Other duplicate test specimens for each of Examples 1and 2 are baked at 190° C. for 100 minutes. One of the test specimensfrom each of the 170° C. and 190° C. bakes are cooled to roomtemperature and their impact peel strength is measured. Other testspecimens from each bake temperature are cooled to ˜−40° C. for onehour, and then immediately tested.

Examples 1 and 2 are separately evaluated for failure mode according toGeneral Motors test specification GMW15200, at a rate of 50 mm/minute.25 mm×100 mm coupons are bent at 90 degrees to form a “T”-shape with theend to be tested 75 mm long. The adhesive is applied between the couponsand cured as described for the peel impact test. Testing is continueduntil the sample breaks apart. The fracture mode is evaluated visually,with cohesive failure being indicated by the lack of visible metal inthe bond area.

Results of the impact peel and T-peel testing are as indicated in Table2.

TABLE 2 Impact Peel Ex. 1 Ex. 2 170° Bake, RT testing, N/mm 25 30 170°C. Bake, −40° C. testing, N/mm 19 30 190° C. Bake, RT testing, N/mm 1921 190° C. Bake, −40° C. testing, N/mm 12 19 T-peel failure mode, 170°C. Bake ND Cohesive T-peel failure mode, 190° C. Bake Cohesive CohesiveND = not determined.

EXAMPLES 3-6

Structural Adhesive Examples 3-6 are prepared and tested in the samegeneral manner as described for Examples 1 and 2, except that impactpeel testing is performed only on samples baked at 190° C. for 100minutes, and T-peel evaluations are done on samples baked at 160° C. for15 minutes, 170° for 30 minutes and 190° C. for 100 minutes. Componentsused to make Examples 3-6 are as indicated in Table 3. Results are asindicated in Table 4.

TABLE 3 Components Ex. 3 Ex. 4 Ex. 5 Ex. 6 Kaneka Kane Ace MX 156 15.015.0 0 15.0 DER 330 Epoxy Resin 21.2 21.2 21.2 21.2 Strutkol 3611 10.610.6 10.6 0 Struktol 3604 10.0 10.0 19.4 10 Advanced Struktol 3604 0 0 010.6 RAM 965 16.7 0 16.7 16.7 RAM B 0 16.7 0 0 Cardura E10 1.1 1.1 1.11.1 Colorants 0.3 0.3 0.3 0.3 Wetting Agent 0.3 0.3 0.3 0.3 SynquestSilane A187 0.7 0.7 0.7 0.7 Amicure CG-1200 4.4 4.4 4.4 4.4 EP796 0.70.7 0.7 0.7 Calcium Oxide 0.5 0.5 0.5 0.5 Dualite E065-135 0 0 0 0 NyadM400 2.2 2.2 2.2 2.2 Calcium Carbonate 12.0 12.0 12.0 12.0 10 mil glassbeads 0.5 0.5 0.5 0.5 Aerosil R805 3.3 3.3 3.3 3.3 Aerosil R202 1.7 1.71.7 1.7

TABLE 4 Impact Peel Ex. 3 Ex. 4 Ex. 5 Ex. 6 170° Bake, RT testing, ND 3334 ND N/mm 170° C. Bake, ND 25 27 ND −40° C. testing, N/mm 190° C. Bake,ND 34 32 ND RT testing, N/mm 190° C. Bake, 28 24 21 31 −40° C. testingN/mm T-peel failure mode, Cohesive Cohesive Cohesive ND 160° C. BakeT-peel failure mode, Cohesive Cohesive Cohesive Adhesive 170° C. BakeT-peel failure mode, Cohesive Cohesive Cohesive Thin Film 190° BakeND—not determined. Thin film failure is a mainly cohesive failure nearthe bond line between the adhesive and substrate.

EXAMPLES 7-9

Structural Adhesive Examples 7-9 are prepared and tested in the samegeneral manner as described for Examples 1 and 2, except that impactpeel testing and T-peel testing are performed only on samples baked at170° C. for 30 minutes. Components used to make Examples 7-9 are asindicated in Table 5. Results are as indicated in Table 6.

TABLE 5 Components Ex. 7 Ex. 8 Ex. 9 Kaneka Kane Ace MX 156 15.0 15.015.0 DER 330 Epoxy Resin 21.2 21.2 21.2 Strutkol 3611 10.6 10.6 10.6Struktol 3604 10.0 10.0 10.0 RAM 965 16.7 16.7 16.7 Cardura E10 1.1 1.11.1 Colorants 0.3 0.3 0.3 Wetting Agent 0.3 0.3 0.3 Synquest Silane A1870.7 0.7 0.7 Amicure CG-1200 4.4 4.4 4.4 EP796 0.7 0.7 0.7 Calcium Oxide0.5 0.5 0.5 Dualite E065-135 0 2 0 Dualite E136-040D 0 0 2 Nyad M400 2.22.2 2.2 Calcium Carbonate 12.0 12.0 12.0 Aerosil R805 3.3 3.3 3.3Aerosil R202 1.7 1.7 1.7

TABLE 6 Impact Peel Ex. 7 Ex. 8 Ex. 9 170° Bake, RT testing, N/mm 36 3330 170° C. Bake, −40° C. testing, 27 26 18 N/mm T-peel failure mode,170° C. Adhesive Cohesive 37% Cohesive Bake

EXAMPLES 10 and 11

Structural Adhesive Examples 10 and 11 are prepared and tested in thesame general manner as described for Examples 1 and 2, except thatimpact peel and T-peel testing are performed on samples baked at 160° C.for 25 minutes, 170° C. for 40 minutes and 190° C. for 60 minutes.Components used to make Examples 10 and 11 are as indicated in Table 7.Results are as indicated in Table 8.

TABLE 7 Parts By Weight Example No. Components 10 11 Kaneka Kane Ace 15612 14.7 DER 330 Epoxy Resin 23.0 0 DER 331 Epoxy Resin 0 19.2 Strutkol3611 11.5 0 Struktol 3604 10.9 20.3 RAM B 14.0 16.6 Cardura E10 1.1 1.1Colorants 0.3 0.3 Wetting Agent 0.3 0.3 Synquest Silane A187 0.7 0.6Amicure CG-1200 3.1 4.3 EP796 0.8 0.7 Dualite E065-135 0 1.5 Talc 0.8 0Nyad M400 2.4 2.2 Calcium Carbonate 8.0 7.9 Aerosil R805 3.6 3.3 AerosilR202 1.8 1.6

TABLE 8 Impact Peel Ex. 10 Ex. 11 160° Bake, RT testing, N/mm 30 31 160°C. Bake, −40° C. testing, 27 29 N/mm 190° C. Bake, RT testing, N/mm 3935 190° C. Bake, −40° C. testing, 36 29 N/mm T-peel failure mode, 170°C. Cohesive Cohesive Bake T-peel failure mode, 190° Bake AdhesiveCohesive

What is claimed is:
 1. A one-component structural adhesive, comprising:A) from 5 to 30 weight percent of one or more rubber-modified epoxyresins, in which at least a portion of the rubber has a glass transitiontemperature of −40° C. or lower; and B) from 5 to 25 parts by weight,per 100 parts by weight of the rubber component of the rubber-modifiedepoxy resin(s), of one or more bisphenol compounds, wherein component B)is pre-reacted with all or a portion of component A) to form an advancedrubber-modified epoxy resin or is present in the form of finely dividedparticles, the structural adhesive further comprising: C) at least 8weight percent of a reactive toughener; and D) one or more epoxy curingagents; wherein the total rubber content is from 8 to 15 weight percentof the structural adhesive and the structural adhesive is curable at atemperature of 80° C. or higher.
 2. The structural adhesive of claim 1further comprising particles of a core-shell rubber.
 3. A methodcomprising applying the structural adhesive of claim 1 between thesurfaces of two metals, and curing the structural adhesive to form anadhesive bond between the two metals.
 4. The structural adhesive ofclaim 1, wherein the bisphenol compound is present in the form of finelydivided particles and constitutes 0.4 to 1.5 weight percent of thestructural adhesive.